Novel Coupled Structures of FeWO₄/TiO₂ and FeWO₄/TiO₂/CdS Designed for Highly Efficient Visible-Light Photocatalysis

A quadrilateral disk-shaped FeWO4 nanocrystal (NC) with an average size of ∼35 nm was prepared via hydrothermal reaction. The obtained dark brown FeWO4 NC with a bandgap (Eg) of 1.98 eV was then coupled with TiO2 to form FeWO4/TiO2 composites. The valence band (VB) of FeWO4 (+2.8 eV vs NHE) was more positive than that of TiO2 (+2.7 eV); thus this system could be classified as a Type-B heterojunction. Under visible-light irradiation, 5/95 FeWO4/TiO2 (by wt %) exhibited remarkable photocatalytic activity: the amount of CO2 evolved from gaseous 2-propanol (IP) and the decomposition rate of aqueous salicylic acid (SA) were, respectively, 1.7 and 2.5 times greater than those of typical nitrogen-doped TiO2 (N-TiO2). This unique catalytic property was deduced to arise from the intersemiconductor hole transfer between the VBs of FeWO4 and TiO2. Herein, several experimental evidence were also provided to confirm the hole-transfer mechanism. To further enhance the catalytic efficiency, double-heterojunctioned FeWO4/TiO2/CdS composites were prepared by loading CdS quantum dots (QDs) onto the FeWO4/TiO2 surface. Surprisingly, the catalytic activity for evolving CO2 from IP was 2.6 times greater than that of bare FeWO4/TiO2 and 4.4 times greater than that of N-TiO2, suggesting that both holes and electrons were essential species in decomposing organic compounds.